In a wireless communications system using a mobile terminal, several types of frequency band signals may be employed having various transmission frequency bands and requiring different transmission power levels.
For example, in case of Universal Mobile Telecommunication System (UMTS) which is one of the third generation standards, there are a variety of subdivided frequency bands such as BAND 1 (transmission frequency band: 1920-1980 MHz) being used in most countries except the U.S.A, BAND 2 (transmission frequency band: 1850-1910 MHz) being used in North and South America, BAND 4, 10 (transmission frequency band: 1710-1770 MHz) being used in some parts of the U.S.A. and Canada, BAND 5 (transmission frequency band: 824-849 MHz) being used in certain parts of the U.S.A and Oceania and BAND 8 (transmission frequency band: 880-915 MHz) being used in Europe, Asia and Oceania. Furthermore, with respect to Global System for Mobile communication (GSM) which is one of the second generation communication standards, there are also a variety of subdivided frequency bands such as GSM 900 (transmission frequency band: 880-915 MHz) and DCS 1800 (transmission frequency band: 1710-1785 MHz) being used in most countries which have adopted GSM; and GSM 850 (transmission frequency band: 824-849 MHz) and PCS 1900 (transmission frequency band: 1850-1910 MHz) being used in some countries including the U.S.A. and Canada.
With the rising need for global roaming services, the demand for roaming services without having to change the mobile terminal has also risen. In efforts to meet the demand, it has become useful to provide a transmission circuit in a mobile terminal that is capable of selectively transmitting a signal in a desired frequency band at a suitable power level. For example, when a user is located in a first region where a first frequency band signal is used for a mobile communications system, the power amplifier circuit of the user's mobile terminal should be able to output the first frequency band signal at a first power level suitable for the region. Similarly, when the user moves to a second region where a second frequency band signal is used for a mobile communications system, the power amplifier circuit should be capable of outputting the second frequency band signal at a second power level suitable for the second region.
Known attempts to provide suitable power levels in different regions and at different frequency bands have certain drawbacks. For example, in one known power amplifier circuit is structured to have two power amplifier cores, each of which consists of a plurality of amplifiers selectively used depending on a type of the frequency band signal to be transmitted; and also to have impedance matching circuits provided at input, intermediate and output stages of the respective amplifier cores. Thus, as the variety of types of frequency band signals for transmission increases, the structure of the power amplifier circuit becomes more complicated, thereby increasing the size and manufacturing cost thereof.
Another known attempt to provide suitable power levels in different regions and at different frequency bands provides isolation between output ports using the parallel resonance circuits. This provides sufficient isolation only when there is a sufficiently large frequency difference between the two types of frequency band signals. If the frequency band signals having a comparatively small frequency difference (e.g., BAND 1 and BAND 2 signals), an effective filtering or isolation between the two paths may not be attained since the characteristics of these two frequency band signals are similar.
Moreover, when three or more types of frequency band signals need be transmitted selectively, the number of output ports of the circuit must be further increased. However, since only one of the output ports may be used to selectively transmit one type of frequency band signals at a time, the number of idle output ports will be concomitantly increased, thereby lowering the efficiency thereof.
What is needed, therefore, is a multi-band power amplifier that overcomes at least the deficiencies of known amplifiers discussed above.